Abstract
A recurrent concern in nature conservation is the potential competition for forage plants between wild bees and managed honey bees. Specifically, that the highly sophisticated system of recruitment and large perennial colonies of honey bees quickly exhaust forage resources leading to the local extirpation of wild bees. However, different species of bees show different preferences for forage plants. We here summarize known forage plants for honey bees and wild bee species at national scale in Denmark. Our focus is on floral resources shared by honey bees and wild bees, with an emphasis on both threatened wild bee species and foraging specialist species. Across all 292 known bee species from Denmark, a total of 410 plant genera were recorded as forage plants. These included 294 plant genera visited by honey bees and 292 plant genera visited by different species of wild bees. Honey bees and wild bees share 176 plant genera in Denmark. Comparing the pairwise niche overlap for individual bee species, no significant relationship was found between their overlap and forage specialization or conservation status. Network analysis of the bee-plant interactions placed honey bees aside from most other bee species, specifically the module containing the honey bee had fewer links to any other modules, while the remaining modules were more highly inter-connected. Despite the lack of predictive relationship from the pairwise niche overlap, data for individual species could be summarized. Consequently, we have identified a set of operational parameters that, based on a high foraging overlap (>70%) and unfavorable conservation status (Vulnerable+Endangered+Critically Endangered), can guide both conservation actions and land management decisions in proximity to known or suspected populations of these species.
Highlights
Many insects, and perhaps wild pollinators in particular, have recently declined at both local and regional scales in north-western Europe and North America [1,2,3,4,5]
From the recorded interactions between bee species and forage plant genera, we summarized and calculated the following measures for each bee species: Total number of known forage plants; Number of forage plants only visited by each individual wild bee species, but not by honey bees; Number of forage plants visited by both the individual wild bee species and honey bee; and MacArthur and Levins [75]’s asymmetrical measure for pairwise niche overlap, the estimate of the extent to which honey bees overlap with wild bee species (Mkj) [76]: Mb kj
We considered only the plant genera visited by wild bees, i.e. the plant genera recorded only for honey bees were excluded, as these plants do not contribute to the diet of wild bee species nor to the pairwise niche overlap measure
Summary
Perhaps wild pollinators in particular, have recently declined at both local and regional scales in north-western Europe and North America [1,2,3,4,5]. Conclusions are dire and mostly support the overall findings by Powney et al [13], showing evidence of a historical decline of pollinators. Faced with this concern, new land use-management policies supporting wild pollinating insects are being implemented, but one of the recurrent concerns has been the potential competition for flower resources between wild bees and managed honey bees [e.g., 14–16]. Competition between species results from exploitation of the same limited resource and is common in plant and animal communities In natural populations, such inter-specific competition leads to niche differentiation over evolutionary time, sometimes observed as character displacement, which acts to minimize competitive overlap between the species [e.g., 17]. Inter-specific interactions such as competition, together with predation, herbivory, mutualism, pathogenic interaction, and parasitism, all contribute to shaping distribution and abundance of organisms [18] structuring ecological communities
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